Growth of the avian granulocyte/monocyte (GM) progenitor cell in the normal chick and in the chick with acute myeloblastic leukemia (AML) will be investigated as a model of human AML. Leukemia will be induced by avian myeloblastosis virus. Replication of the GM progenitor cell will be assayed in vitro in semisolid cultures. Experiments are designed to determine whether there are quantitative or qualitative alterations in the factor (colony stimulating factor) which stimulates division of this progenitor cell. This factor will be obtained from serum and from medium conditioned by macrophages, lymphocytes and bone cells. A primary question is whether such alterations would contribute to disease progression. Another central question is whether the leukemic cell is altered in its responsiveness to this factor.